add gas station

greedy_methods/gas_station.py

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+"""
+Task:
+There are n gas stations along a circular route, where the amount of gas
+at the ith station is gas_quantities[i].
+
+You have a car with an unlimited gas tank and it costs costs[i] of gas
+to travel from the ith station to its next (i + 1)th station.
+You begin the journey with an empty tank at one of the gas stations.
+
+Given two integer arrays gas_quantities and costs, return the starting
+gas station's index if you can travel around the circuit once
+in the clockwise direction otherwise, return -1.
+If there exists a solution, it is guaranteed to be unique
+
+Reference: https://leetcode.com/problems/gas-station/description
+
+Implementation notes:
+First, check whether the total gas is enough to complete the journey. If not, return -1.
+However, if there is enough gas, it is guaranteed that there is a valid
+starting index to reach the end of the journey.
+Greedily calculate the net gain (gas_quantity - cost) at each station.
+If the net gain ever goes below 0 while iterating through the stations,
+start checking from the next station.
+
+"""
+from dataclasses import dataclass
+
+
+@dataclass
+class GasStation:
+    gas_quantity: int
+    cost: int
+
+
+def get_gas_stations(
+    gas_quantities: list[int], costs: list[int]
+) -> tuple[GasStation, ...]:
+    """
+    This function returns a tuple of gas stations.
+
+    Args:
+        gas_quantities: Amount of gas available at each station
+        costs: The cost of gas required to move from one station to the next
+
+    Returns:
+        A tuple of gas stations
+
+    >>> gas_stations = get_gas_stations([1, 2, 3, 4, 5], [3, 4, 5, 1, 2])
+    >>> len(gas_stations)
+    5
+    >>> gas_stations[0]
+    GasStation(gas_quantity=1, cost=3)
+    >>> gas_stations[-1]
+    GasStation(gas_quantity=5, cost=2)
+    """
+    return tuple(
+        GasStation(quantity, cost) for quantity, cost in zip(gas_quantities, costs)
+    )
+
+
+def can_complete_journey(gas_stations: tuple[GasStation, ...]) -> int:
+    """
+    This function returns the index from which to start the journey
+    in order to reach the end.
+
+    Args:
+        gas_quantities [list]: Amount of gas available at each station
+        cost [list]: The cost of gas required to move from one station to the next
+
+    Returns:
+        start [int]: start index needed to complete the journey
+
+    Examples:
+    >>> can_complete_journey(get_gas_stations([1, 2, 3, 4, 5], [3, 4, 5, 1, 2]))
+    3
+    >>> can_complete_journey(get_gas_stations([2, 3, 4], [3, 4, 3]))
+    -1
+    """
+    total_gas = sum(gas_station.gas_quantity for gas_station in gas_stations)
+    total_cost = sum(gas_station.cost for gas_station in gas_stations)
+    if total_gas < total_cost:
+        return -1
+
+    start = 0
+    net = 0
+    for i, gas_station in enumerate(gas_stations):
+        net += gas_station.gas_quantity - gas_station.cost
+        if net < 0:
+            start = i + 1
+            net = 0
+    return start
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()